1. Field of the Invention
The present invention relates to an isolating valve of the ball valve type wherein a ball means rotatable by means of a rotating shaft device floats in a body provided with upstream and downstream pipe stubs, the downstream face of the spherical circumference of said ball means mating with a complementary sphere-bent annular body sealing surface, a pressure difference building up between the upstream and the downstream sides of said ball means as said ball means is moved into the closed position, said ball means and said sealing surface forming a primary sealing system which is substantially concentric with the axis of said pipe stubs.
2. Description of Related Art
A variety of isolating valves of different designs such as gate valves, screw-down stop valves, plug valves and check valves are well known. For the design and operation of high-pressure pipelines, gate valves and plug valves are usually employed for small and medium pipeline diameters up to approximately DN 400, whereas ball valves are mostly used for large diameters from DN 500 to DN 1200. The angle of rotation of the latter is only 90.degree.. For sealing, in addition to the load brought to bear by the ball, the pressure difference across an area equivalent to the cross-sectional area of the pipe in the case of floating balls or across an area equivalent to the cross-sectional area of the sealing ring in the case of balls supported by two sealing rings becomes effective. It is therefore an advantage of ball valves over gate valves that the force required for moving the obturator is substantially smaller allowing even large ball valves to be opened or closed by non-motorized manual actuators.
The sealing systems used for such isolating valves are exposed to the fluid passing through the pipeline system in which said valves are incorporated, the exposure depending on the valve design, and may thence be damaged mainly by solid particles carried by the fluid, metal sealing elements being, by the nature of the material, more resistant to wear than non-metallic sealing elements. Ball valves with non-metallic main sealing systems are therefore less fit for pipelines or pipework, if the fluid piped therethrough may carry solid particles.
In view of the complex geometry of ball valve sealing elements, the use of metal seals for ball valves implies complex manufacturing operations and the application of a relatively high force on the seal for tight sealing. Experience has shown that for ball valves, sealing systems comprising a primary metal seal and a secondary non-metallic seal are relatively resistant to wear.
For various repair and inspection operations, it is necessary to ensure reliably in the field that the seal provided by such an isolating valve is absolutely tight. Block and bleed systems are usually used therefor.
It is standard practice to ensure the tightness of the seal by venting the body of such a valve using a drain line. Both in the case of balls supported by two sealing rings and in the case of floating balls, sealing rings on mobile supports following the movement of the ball of such an isolating valve are necessary for so ensuring the tightness of the seal by venting the body of such a valve. The force required for pressing the ball tightly against such a sealing system is produced by springs and the exposure of the annular surfaces to pressure. In the case of floating ball valves, forces acting upon the downstream sealing system are very high because of the pressure difference across the entire valve bore cross-sectional area, whereas the upstream sealing support system must accomodate relatively large axial movements.
The use of such mobile seal supports necessitates two independent sealing systems. One such sealing system is required between the ball and the seal support and a second seal is necessary between said seal support and the body of the valve. The manufacture of such mobile sealing systems for ball valves is complex and the ingress and deposition of solid particles mainly in the inner sealing system frequently interferes with the functioning of such a valve primarily as regards the block and bleed system. The complexity of such conventional sealing systems, increased further by a block and bleed system, has been the cause of numerous valve failures necessitating costly repairs or even the replacement of valves.